The Sootopolis Gym is a Water-type Gym, and the eighth gym in the Hoenn region. You can reach it after dealing with the legendaries in the Cave of Origin.

Prepare

Many of the pokémon that you will encounter in this gym are weak to Grass-type moves, as well as Electric-type moves. One exception in Ruby and Sapphire is that there is one trainer who has a Lombre, which is weak to Flying, Poison, and Bug moves. In Emerald, the Gym Leader has a Kingdra, which is only weak to Dragon moves.

Furthermore, a couple of the Gym Leader's pokémon have Ice moves, which Grass-type pokémon are weak to, so be sure to bring other types, such as Electric-type pokémon, as well.

Go Through the Gym

In this gym, you have to walk across the ice to reach the gym leader. The ice cracks when you step on it, so if you step on it again, it will break, and you will fall down below to an area where there are trainers. You have to go to the bottom of that area to go up the ladder to the area above.

Ruby and Sapphire

To get through the gym in Ruby and Sapphire, step onto the first piece of ice, then go right, up, left, left, up, and right. This makes stairs appear. If you fall down in the first area, you might encounter Beauty Olivia, who has a Lombre, and Lass Crissy, who has two Wailmers.

Go up the stairs to the second area. Step on the first piece of ice, then go all the way to the left, then go all the way up, then go right, right, down, right, right, right, down, right, all the way up, then go to the left until the stairs appear. If you fall down in this second area, you might encounter Beauty Tiffany, who has Carvanha, Wailmer, and Sharpedo, and Pokéfan Marissa, who has Azurill, Marill, and Azumarill.

Go up the stairs to the third area. Step on the first piece of ice, then go right until you hit the rock, then go up, right, right, down, right, all the way up, left, left, down, left, up, left, down, down, left, left, down, left, up, left, down, left, up, left, all the way up, right, down, right, up, right, down, right, up, and right. Then you can go up the stairs to the gym leader. If you fall down in this third area, you can encounter Beauty Bridget who has Azumarill; Lady Brianna who has Seaking; Beauty Connie who has Goldeen and Wailmer; and Lass Andrea who has Luvdisc.

After you go all the way to the top of the area, you can battle Wallace.

Emerald

To get through the gym in Emerald, step onto the first piece of ice, then go left, up, right, right, up, and left. This makes the stairs appear. If you fall down in the first area, you might encounter Beauty Connie, who has a Goldeen, and Lass Andrea, who has a Luvdisc.

Go up the stairs to the second area. Step on the first piece of ice, then go all the way to the left, then go all the way up, then go right, right, down, right right, down, all the way right, all the way up, and then go left until the stairs appear. If you fall down in this area, you might encounter Lass Daphne, who has two Luvdiscs, and Pokéfan Annika, who has two Feebas. Daphne and Annika might join together in a Double Battle against you.

Go up the stairs to the third area. Step on the first piece of ice, then go left, left, up, left, down, all the way left, all the way up, then right, down, right, right, up, right, down, right, down, right, down, right, up, right, right, down, right, all the way up, left, left, down, left, up, and go left until the stairs appear. If you fall down in this area, you might encounter Beauty Tiffany, who has Carvanha and Sharpedo, and who might do a double battle with Crissy; Lass Crissy, who has Goldeen and Wailmer, and who might do a double battle with Tiffany; Beauty Olivia, who has Clamperl, Corphish, and Lombre, and who might do a double battle with Bethany; Pokéfan Bethany, who has Azurill, Marill, and Azumarill, and who might do a double battle with Olivia; Beauty Bridget, who has Azumarill, and who might do a double battle with Brianna; and Lady Brianna, who has Seaking, and who might do a double battle with Bridget.

After you go all the way to the top of the area, you can battle Juan.

Gym Leader Wallace (Ruby and Sapphire)

Wallace has a level 40 Luvdisc, a level 42 Whiscash, a level 40 Sealeo, a level 42 Seaking, and a level 43 Milotic.

All of Wallace's pokémon know Water Pulse, which is a Water-type attack that has a chance of causing confusion.

The Luvdisc can be a challenge because it knows Sweet Kiss, which causes Confusion, and Attract, which causes infatuation. These status ailments can slow down the battle considerably. Switch out a pokémon who is suffering from one of these conditions and it will be cured.

The Whiscash is Water/Ground, so it is very weak to Grass moves. In addition to Water attacks, it can use Earthquake.

The Sealeo knows an Ice attack, Aurora Beam, as well as Water Pulse. Grass-type pokémon are not recommended against Sealeo because they are weak to the Ice attack. Sealeo also knows Encore, which forces your pokémon to use the same move several turns in a row.

The Seaking knows Horn Drill, which has a 30% chance of doing a one-hit KO against your pokémon. In other words, it has a 30% chance of making your pokémon faint.

The Milotic knows the Dragon-type attack Twister. It also knows the Ice attack Ice Beam. In addition, it knows the healing move Recover. Don't use a Dragon-type pokémon against Milotic, because it will be weak to the Dragon-type attack. In addition, don't use a Grass-type pokémon, because it will be weak to Ice Beam.

Gym Leader Juan (Emerald)

Juan has a level 41 Luvdisc, a level 41 Whiscash, a level 43 Sealeo, a level 43 Crawdaunt, and a level 46 Kingdra.

All of Juan's pokémon know Water Pulse, which is a Water-type attack that has a chance of causing confusion.

The Luvdisc can be a challenge because it knows Sweet Kiss, which causes Confusion, and Attract, which causes infatuation. These status ailments can slow down the battle considerably. Switch out a pokémon who is suffering from one of these conditions and it will be cured.

The Whiscash is Water/Ground, so it is very weak to Grass moves. In addition to Water attacks, it can use Earthquake.

The Sealeo knows an Ice attack, Aurora Beam, as well as Water Pulse. Grass-type pokémon are not recommended against Sealeo because they are weak to the Ice attack. Sealeo also knows Encore, which forces your pokémon to use the same move several turns in a row.

The Crawdaunt knows Taunt, which forces your pokémon to only use attacks, and not status-affecting moves.

The Kingdra is Water/Dragon, so it's only weak to Dragon moves, and it's strong against Fire, Ice, Steel, Water, Electric, and Grass moves. It has a Water attack and Ice attack. It also knows Rest, a healing move.

After You Defeat the Gym Leader

After the battle, you get the Rain Badge, which causes all traded pokémon to obey you, and it allows you to use Waterfall outside of battle. You also get TM03 Water Pulse.

Now that you have all eight gym badges, you should go to Victory Road. To get there, use HM07 to teach Waterfall to one of your pokémon, then go south from Mossdeep City, east from Route 128, and then use Waterfall to go up. You will reach Ever Grande City, where you will find the entrance to Victory Road.